1. Field of the Invention
The invention relates to a high refractive index plastic, produced by reacting an alkylpolythiol with an alkylpolythiol ester of (meth)acrylic acid, under the conditions of radical polymerization.
2. Discussion of the Background
A high refractive index is very advantageous for plastics which are to be used in optical elements.
A high refractive index of the material can be achieved, e.g., by incorporating sulfur atoms into the plastic. One means of introducing sulfur into a polymer chain, is by thiol addition to an organic isocyanate. Thus, in EP-A 235,743, 330,363 and 351,073 high refractive index polymers are proposed which are produced from polythiols and polyisocyanates, or from sulfur-containing polyisocyanates and polyisothiocyanates.
However, in general, the optical dispersion of such polyurethane derivatives is excessive. An exception is the case of polymers with halogen-containing aromatic dithiols. However, the halogen content presents drawbacks in processing and toxicity. Thiol-ene addition can also be used to produce optical materials, e.g. in the form of addition products of polythiols to multiply unsaturated compounds. An example of this is the use of dimethacrylates of dihydric alcohols (see, e.g., EP-A 437,374 and 284,374). However, the refractive index of such materials is insufficiently high. The polymers claimed in EP 394,495 have high indices of refraction (&gt;1.65), but the aromatic diene (4,4,'-bis((meth)acryloylthio)diphenyl sulfide) causes a high dispersion of light.
The sulfur-containing polymers of the state of the art are not satisfactory for use in optical components, either as to their optical properties or their mechanical properties.
Accordingly, the underlying problem of the invention is to devise transparent, colorless plastics with a high refractive index and a high Abbe number.
It has been discovered, in connection with the present invention, that transparent, colorless polymers, with very good optical properties can be produced by reacting polythiols with multifunctional alkyl thio(meth)acrylates under conditions of simultaneous polymerization and polyaddition.